1. Field of the Invention
The present invention relates to a method and device for controlling the heating of a battery and more particularly to a method and device for controlling the selective heating of an automobile battery using the excess electrical capacity of the vehicle electrical power generation subsystem comprising an alternator, rectifier and regulator.
2. Description of the Prior Art
By way of background, it is appropriate to initially provide a brief description of the modern vehicle electrical system. It is helpful to consider such systems as being divided into three major parts: the electrical power generation subsystem, the storage battery and the electrical loads. The generation subsystem usually consists of a poly-phase alternator, a rectifier and a voltage regulator.
The actual electrical power for the electrical loads may come from two sources: the storage battery and/or the generation subsystem. When the engine of the vehicle is not running the battery supplies the necessary power. When the engine is running, i.e. the alternator is turning, the generation subsystem supplies the power, including that which is necessary to recharge the battery. These two sources are electrically connected in parallel such that they interact to determine the voltage level of the system that is available to the loads.
Both the battery and the generation subsystem have limits to the amount of power they can supply. The battery supplies only so much power for so long before its energy reserve is depleted and its voltage drops to a point below what is useful. The generation subsystem can supply power for as long as the alternator is turning. The amount of available power is related to the alternator RPM, the temperature of the generation components and the voltage at which the power is supplied.
When the engine is not running, the battery supplies power at a voltage that is a function of the battery's state of charge and the rate at which the battery is supplying current. On the other hand, when the engine is running, the generation subsystem supplies the power at a voltage that is determined by the regulator. This voltage value, or set point voltage, is such that the generation subsystem will act to charge the battery. However, if the generation subsystem cannot meet the electrical load's demand at the set point voltage plus the additional demand required to charge the battery, the system voltage will drop to a value less than the set point voltage. This is often referred to as unregulated operation.
In the case where the generation subsystem's capacity is exceeded, the battery in combination with the alternator/rectifier and the presently powered electrical loads reach a point of balance that determines the system voltage available to the loads. This balance is the result of the general current/voltage relationship of the electrical components.
For the battery, as the system voltage drops below the set point voltage the charge current into the battery also drops. If the voltage continues to fall below the point where the battery will charge, the battery begins to discharge to make up for the lack of available power in the generation subsystem. For the unregulated alternator/rectifier, as the system voltage drops the amount of current that it can provide will increase. These two trends alone will act to cause the unregulated voltage to settle to a value below the set point called the balance voltage. Generally, loads require less current at lower voltages, which aids in the balancing.
A regulator controls the voltage available to the loads when the generation system has sufficient capacity to meet the electrical power demand. In modern electrical system, this is commonly accomplished by a closed-loop control system that varies the average electrical current thru the alternator's field coil which in turn varies the rotor's magnetic field and hence the alternator's output current. The voltage at the loads is a function of the alternator output current across the loads of the vehicle. The regulator monitors this voltage and varies the field coil current to control the voltage to the set point.
As the characteristics that control generation subsystem capacity and vehicle electrical load vary, the primary function of the regulator is to respond in such a manner as to present suitable voltage to the loads and battery. This means being able to quickly reduce the field current when the electrical demand is reduced, thereby preventing the overvoltage of the loads, and being able to increase the field current in a controlled fashion when the demand is increased.
The regulator set point is chosen such that the voltage is high enough that it will charge the battery directly from the rectified alternator output. In this way, the battery acts as another load on the vehicle. The set point voltage must also be low enough so that the rest of the vehicle loads will not exceed their rated maximum operation voltage.
It has long been observed that batteries charge less effectively and have reduced available power at lower temperatures. That is, the charge acceptance and discharge performance at such temperatures are said to be reduced. Consequently, many techniques have been employed by vehicle operators, automobile manufacturers and battery manufacturers to ensure that the temperature of the batteries are at desired levels upon starting and during operation when the underhood temperature is not sufficient to warm the battery. Various charge management systems that are based on heating have been employed having heating elements both within and without the internal confines of batteries. U.S. Pat. No. 3,723,187 issued on May 27, 1973 to Toydoka is an example of an internal system while U.S. Pat. No. 4,081,737 issued Mar. 28, 1978 to Mijahara is an example of an external system.
One problem with such heating systems is the need to employ a power source to heat the battery. The power source may be on or off-board the vehicle in which the battery is located. When heating is to be accomplished for vehicle batteries in a moving vehicle, it is clear that the source supplying the energy necessary to heat the battery must be on board the vehicle. U.S. Pat. No. 4,025,861 issued May 24, 1977 to Godard describes a device for heating a battery only when the voltage of the battery reaches a certain predetermined value which depends upon the battery's state of charge and on battery temperature. U.S. Pat. No. 4,222,000 issued Sept. 9, 1980, describes a battery system in which the alternator driven by the vehicle engine supplies an alternating current directly to the battery, the internal resistance thereof causing heat to be dissipated which raises the temperature of the electrolyte.
Alternatively, some vehicle manufacturers have employed a charge management strategy which involves raising the regulator's set point voltage when the underhood temperature is low to compensate for the reduced charge acceptance of the battery. As the charging voltage is raised for a constant battery temperature, the rate of charge acceptance increases. Temperature sensors for such strategies have been located in the alternator/regulator package, in a remote regulator module and in direct contact with the battery.
A major deficiency of this charge management strategy is that when the battery is very cold, the voltage may not be able to be raised high enough to properly charge the battery without damaging other electrical components. Furthermore, when the temperature sensor is located in the alternator/regular package or in a remote regulator module, the sensor heats much more rapidly than the battery does and to a level that is not necessarily the same as the battery. Thus, there is a reduction in the set point voltage; however, higher voltage is still needed to compensate for the low charge acceptance capability of the battery. Also, this strategy does nothing to directly increase battery temperatures which would result in improved charge acceptance and discharge performance.